1. Field of the Invention
The present invention relates to flyback transformers and, more particularly, to a flyback transformer used in a PWM (Pulse Width Modulation) controlled voltage generator circuit for providing a high voltage to a cathode ray tube.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a high voltage generator circuit including a flyback transformer and which is the motivation for the development of the present invention. The anode of a diode 14 is connected to the primary winding of a flyback transformer 12 for use in a high voltage generator circuit 10 while the cathode of the diode 14 is connected to the drain of an FET (Field-Effect Transistor) 16 functioning as a switching element. The source of the FET 16 is connected to one end of a resistor 18 and the other end of the resistor 18 is grounded. A diode 20 is connected in parallel with the serial circuit of the diode 14, the FET 16, and the resistor 18. The cathode of the diode 20 is connected to the anode of the diode 14, while the anode of the diode 20 is grounded.
The serial connection of a resonance capacitor 22 and a diode 24 is connected in parallel with the diode 20. One end of the resonance capacitor 22 is connected to the anode of the diode 14, while the other end of the resonance capacitor 22 is connected to the cathode of the diode 24. The anode of the diode 24 is grounded. The junction of the resonance capacitor 22 and the diode 24 is connected to the anode of another diode 26, and the cathode of the diode 26 is connected to the primary winding of the flyback transformer 12 through a ringing suppressing circuit 28. The ringing suppressing circuit 28 comprises a capacitor 30, a resistor 32, and an inductor 34. A power supply +B is connected to the junction between the diode 26 and the ringing suppressing circuit 28. The junction between the diode 26 and the ringing suppressing circuit 28 is grounded through a capacitor 36 and a capacitor 38.
A signal for controlling the FET 16 in an on and off operation is fed to a gate of the FET 16 from a PWM (Pulse Width Modulation) control circuit 40. The control PWM circuit 40 receives a voltage which is obtained by voltage dividing the output voltage across the secondary winding of the flyback transformer 12. The control signal for controlling the FET 16 is formed based on the divided voltage and an input horizontal drive voltage. The junction of the FET 16 and the resistor 18 is connected to a protective circuit in the PWM 40, and an excessive current flowing through the circuit is detected by the protective circuit.
FIG. 2 show waveforms of voltages at representative points of the high voltage generator circuit 10. As shown, FIG. 2(a) shows a waveform of a signal for controlling the FET 16, and FIG. 2(b) shows a waveform of the voltage at point A in FIG. 1. FIG. 2(c) shows a waveform of a current flowing in the primary winding of the flyback transformer 12. When the FET 16 is turned on at t0, a current flows from the power supply +B through the primary winding of the flyback transformer 12, the diode 14, the FET 16, and the resistor 18. The primary winding of the flyback transformer 12 stores electromagnetic energy when this current flows.
When the FET 16 is turned off at time t1, a current flows from the primary winding of the flyback transformer 12 through the resonance capacitor 22, and the diode 26, and the primary winding of the flyback transformer 12 and the resonance capacitor 22 starts resonating, thereby generating a flyback pulse as shown in FIG. 2(b). The flyback pulse is maximized at the moment the electromagnetic energy stored in the flyback transformer is entirely converted into electrostatic energy in the resonance capacitor 22.
When the electromagnetic energy stored in the primary winding of the flyback transformer 12 is entirely transferred to the resonance capacitor 22, a reverse currents flows through a path along the diode 24, the resonance capacitor 22, and the primary winding of the flyback transformer 12. The electrostatic energy in the resonance capacitor 22 is transformed back into electrostatic energy in the primary winding of the flyback transformer 12. Charge stored in a stray capacitance of the FET 16 is blocked by the diode 14, and does not flow into the primary winding of the flyback transformer 12.
The voltage at point A returns to zero at time t2 when the flyback pulse ends. The diode 20 is turned on, permitting a current to flow from ground to the primary winding of the flyback transformer 12. The voltage at point A rises when this current flows, and reaches the voltage of the power supply +B. At time t3, the diode 20 is turned off, and the current becomes zero. A current from the power supply +B attempts to flow into the resonance capacitor 22, but a current prevention clamping circuit comprising the diodes 24 and 26 clamps the voltage across the resonance capacitor 22 to the voltage of the power supply +B. No current flows from the primary winding of the flyback transformer 12 to the resonance capacitor 22. When the FET 16 is turned on at time t4, a current flows from the power supply +B to the primary winding of the flyback transformer 12, and the state of the circuit is the same as that at time t0. The high voltage generator circuit functions repeating the above-referenced operation. The flyback transformer 12 raises the flyback pulse in voltage level, and provides a high voltage from the secondary winding thereof.
When the current drops to zero at time t3, resonance takes place between the primary winding of the flyback transformer 12 and capacitance of the circuit including the stray capacity present in the FET 16. Ringing is thus generated from time t3 to time t4. The ringing suppressing circuit 28 is used to control ringing.
In the high-tension generator circuit 10, inductance Lp of the primary winding of the flyback transformer 12 satisfies the following condition:
Lpxe2x89xa6Ebxc3x97Ts/Ipp 
where Eb is the voltage of the power supply +B, Ts is a duration of time from the end of the flyback pulse to the start of the next flyback pulse, and Ipp is an input current to the flyback transformer 12. Under this condition, the input current Ipp must meet the permissible current of the FET 16. The conventional flyback transformer 12 must satisfy these conditions and is designed to provide a required output voltage from the secondary winding thereof.
Magnetic flux density Bmax generated in the core of the flyback transformer 12 is Bmax=Lpxc3x97Ipp/N1xc3x97S, where N1 is the number of the primary winding coils, and S is the cross-sectional area of the core of the primary winding. Given a constant voltage Eb of the power supply +B, the input current Ipp is maintained low, the core is reduced in size, and the flyback transformer 12 is made compact. A lower input current Ipp leads to a lower power consumption.
Accordingly, it is an object of the present invention to provide a compact and low-power consumption flyback transformer which reduces an input current flowing thereinto.
A flyback transformer of the present invention for use in a PWM controlled high voltage generator circuit, includes a layer-wound secondary winding having the number of layers equal to or larger than six.
In a preferred embodiment, a diode is configured with the cathode thereof connected to one end of the secondary winding and with the anode thereof grounded.
The flyback transformer is preferably used in a PWM controlled high voltage generator circuit having an operation frequency of 70 kHz or higher.
The number of turns of the secondary winding may be not more than 2500.
Given the same number of winding turns, an increase in the number of layers of layer-wound secondary winding narrows the width across the secondary winding and reduces distributed capacitance of the flyback transformer. As distributed capacitance becomes lower, the frequency of ringing becomes higher, and the pulse width of the flyback pulse becomes narrower. This arrangement expedites the timing of stopping the ringing, thereby lengthening the on-time of the switching element. As the on-time of the switching element is lengthened, the gradient of the waveform of a current flowing through the primary winding of the flyback transformer is reduced, and the input current Ipp is also reduced.
In a flyback transformer circuit in which a diode is connected between one end of the secondary winding and ground (D0 diode method), an alternating current zero point which is grounded in terms of alternating current occurs approximately at the center of the secondary winding. Opposite pulses of negative and positive occur in the vicinity of the alternating current zero point. The alternating current zero point is determined by a distributed capacitance between the primary winding and the secondary winding, and a distributed capacitance between the winding and a ground area. Since the distributed capacitance changes depending on the separation between each of the windings and the ground area, the alternating current zero point is generated at a position offset from the center of the winding.
If the winding width is narrowed by increasing the number of layers of layer winding in the flyback transformer operating on the D0 switching method, the separation between each of the windings and the ground area becomes small, and the alternating current zero point approaches the center of the winding. In this way, the winding widths of portions of the winding which generates positive and negative pulses become close to each other, and the resonance frequencies of the positive pulse and the negative pulse become close, thereby reducing loss involved in resonance.
These and other objects, features and advantages will become apparent from the discussion of the following embodiments of the present invention and the accompanying drawings.